Example of a type-safe api in Crystal http://blog.tarn-vedra.de/posts/phantom-types-for-safe-apis.html

typesafe-api.cr

class Connection
  class Connected
    def disconnect
      Disconnected.new
    end
  end

  class Disconnected
    def connect
      Connected.new
    end
  end

  def self.new
    Disconnected.new
  end
end

x = Connection.new
# x.disconnect       # compile-time error

x = x.connect
# x.connect          # compile-time error
x = x.disconnect